In digital communications transmitters, a modulator is employed to modulate the amplitude, phase and/or frequency of a carrier signal capable of being transmitted over a communications medium (e.g., air, space, wire, cable, fiber optics) to a remote receiver. How the modulator modulates the carrier signal depends on the type of modulation scheme that is used. In some communications systems, simple modulation schemes like frequency-shift keying (FSK) or phase-shift keying (PSK) are used and the modulator need only modulate the phase or frequency of the carrier signal. With those types of modulation schemes, the resulting modulated carrier signal has constant amplitude, i.e., has a constant “envelope.”
In an effort to increase spectral efficiency, many communications systems such as, for example. Bluetooth EDR (extended data rate). Wi-Fi, WiMAX (Worldwide Interoperability for Microwave Access), EDGE (Enhanced Data rates for GSM Evolution), W-CDMA (Wideband Code Division Multiple Access), and Long-Term Evolution (LTE), require modulators that operate according to complex modulation schemes in which both the amplitude and angle of the carrier signal are modulated. When these “non-constant envelope” modulation schemes are used in conventional quadrature-modulator-based transmitters, output power back-off (OBO) must be applied to prevent signal distortion. OBO prevents signal distortion by ensuring that the power amplifier (PA) of the quadrature-modulator-based transmitter operates in its linear region of operation for the full dynamic range of output powers the transmitter must be configurable to transmit. Unfortunately, linear PAs are not very energy efficient and OBO consequently results in a sacrifice of energy efficiency for linearity.
One way of avoiding the linearity versus efficiency tradeoff is to use a polar modulation transmitter, rather than a quadrature-modulator-based transmitter. In a polar modulation transmitter, modulation is performed in the polar domain using separate amplitude modulating and angle modulating signals. The angle modulating signal has a constant amplitude and is used to generate a constant-envelope phase-modulated carrier signal. The constant-envelope phase-modulated carrier signal is coupled to the signal input of the polar modulation transmitter's PA. Because it has a constant envelope, OBO is not required and the PA can be configured to operate as an energy-efficient nonlinear PA without the risk of signal peak clipping. Typically, the PA is implemented as a switch-mode PA, which consumes most of its power only during times when it is switching. As the phase-modulated carrier signal is applied to the signal input of the PA, the amplitude modulating signal containing the signal envelope is applied to the power supply port of the PA. When configured in this manner, the switch-mode PA effectively operates as a modulator, modulating the phase-modulated carrier signal by the amplitude information in the amplitude-modulated power supply signal to produce the desired non-constant envelope carrier signal at its output.
Although the polar modulation transmitter is more energy efficient than the quadrature-modulator-based transmitter, it does have its own limitations. One limitation relates to a phenomenon known as “bandwidth expansion.” Some modulation schemes produce signals having signal trajectories that pass through or very close to the origin in the complex signal plane. When converted to and processed in the polar domain, these types of signal trajectories have abrupt changes in phase. In fact, for signal trajectories that pass directly through the origin, an instantaneous phase change of 180° occurs. Abrupt phase changes correspond to high-frequency events in the polar domain and are highly undesirable since they can exceed the tuning bandwidth capability of the phase modulator used to generate the phase-modulated carrier signal.
Another limitation of the polar modulation transmitter is that at low output power levels the PA can become incapable of operating as a switch. To what extent this is a problem depends on the modulation scheme being used and on noise limitation and modulation requirements of the controlling communications standard. In general, modulation schemes that produce amplitude modulating signals having low magnitudes tend to be the most problematic.
It would be desirable, therefore, to have modulator methods and apparatus that combine the best attributes of quadrature-modulator-based and polar modulation transmitters yet avoid their various drawbacks and limitations.